This invention relates to a catalyst, its method of preparation and its use in converting nitrogen oxides in exhaust gases. More particularly, the invention relates to a catalyst useful for converting nitrogen oxides in exhaust gases from furnaces, coke works and boilers, by adding a reducing gas, such as ammonia, to the exhaust gas and reacting the nitrogen oxides in the presence of the catalyst to form nitrogen. The catalyst is comprised mainly of a double oxide of niobium and iron.
Atmospheric pollution containing nitrogen oxides (NO.sub.x) from exhaust gases discharged into the atmosphere has been increasing its noxious effects year after year in the vicinity of large industrial centers. Finding a solution to the problem of atmospheric pollution is a task of great importance which should be carried out as soon as possible.
The method of selective reduction of nitrogen oxides by dry means converts harmful nitrogen oxides to inert nitrogen gas. The method is carried out with a reducing gas, such as ammonia. The method is readily carried out and results in the removal of nitrogen oxides to a high degree. In comparison with methods based on humid conditions, selective reduction by dry means is less costly, does not require the treatment of residual liquids and requires less maintenance. Because of these advantages, selective reduction by dry means is close to its final stage of development for practical application.
Iron oxides are used as catalysts in selective reduction by dry means. These materials are relatively cheap and in abundant supply. Vanadium oxides, titanium oxide, etc. are also known for use in the method, there being many publications up to the present time.
Since the known method of selective reduction by dry means still presents many problems, there is ample room for further study and research. Among the significant remaining problems is the decrease in catalytic activity when the known catalysts are used to treat exhaust gases containing sulphur oxides. Another significant problem, especially in the case of iron catalysts, is an undesired catalytic side reaction, namely, acceleration of oxidation of the ammonia that is present to convert the nitrogen oxides to inert nitrogen. Oxidation of the ammonia further increases the amount of nitrogen oxides. Furthermore, in the case of various multiple oxides (or mixtures of oxides) as catalysts, catalytic activity has been found to be dependent on thermal conditions, and this has been found to restrict the temperature range in which the catalyst can be employed.